The present invention relates in general to mobile radiocommunications systems.
More particularly, the present invention relates to location services that can be implemented in such systems in order to determine the geographical positions of the users of mobile stations.
For example, in the global system for mobile communications (GSM), such services are referred to as location services (LCS) and are defined in particular in specification 3GPP TS 43.059 published by the third generation partnership project (3GPP).
In general, in such systems, a distinction is drawn between circuit mode services and packet mode services. For example, in the GSM system, services in packet mode correspond to general packet radio service (GPRS) functionality. Circuit mode services and packet mode services lead to different circuit architectures and to different implementations for location services.
By way of example, the architecture of a GSM/GPRS system is summarized in FIG. 1. Various types of entity or equipment can be seen:                a base station subsystem (BSS) including base transceiver stations (BTS) in communication with mobile stations (MS), and base station controllers (BSC); and        a serving GPRS support node (SGSN) in communication with the BSS, and a gateway GPRS support node (GGSN) in communication with the SGSN and with external networks (not shown).        
More generally, a distinction is drawn between a radio access network (RAN) and a core network (CN).
Furthermore, the functionalities of the BSS include functionalities that are common to circuit mode services and packet mode services, and functionalities that are specific to packet mode services. The functionalities specific to packet mode services correspond in particular to an entity of the packet control unit (PCU) type. For a description of the functions of a PCU, reference can be made in particular to specifications 3GPP TS 23.060. As shown in FIG. 1, PCU type equipments can be located in particular between BSC type equipments and SGSN type equipments, in which case equipment of the PCU type can control a plurality of equipments of the BSC type, as shown in FIG. 1.
In the layer architecture used to describe such systems, distinctions are drawn at the “Um” interface between MS and BSS as follows in order of increasing level:                a radio frequency (RF) GSM layer;        a medium access control (MAC) layer; and        a radio link control (RLC) layer.        
Similarly, at the “Gb” interface between the BSS and the SGSN, the following can be distinguished:                an L1bis layer;        a network service layer; and        a BSS GPRS protocol (BSSGP) layer.        
Between MS and SGSN, a higher layer or logical link control (LLC) layer enables LLC frames to be exchanged that are made from data units of higher level. In LLC frames, these data units are referred to as LLC protocol data units (LLC-PDU).
The LLC-PDU data units are then segmented in the MAC/RLC layer so as to form blocks referred to as RLC data blocks. Thereafter the RLC data blocks are put into the format required for transmission over the “Um” interface in the physical layer.
The “network service” layer is defined in particular in specification 3GPP TS 48.016. It is recalled that the functions implemented by the network service layer include in particular managing the identifiers of virtual connections used for communication over the “Gb” interface in the BSS GPRS protocol (BSSGP) as defined in particular in specification 3GPP TS 48.018. It is also recalled that a virtual connection is identified by means of a network service entity (NSE) identifier (NSEI) and by means of a BSSGP virtual connection identifier (BVCI).
In addition, higher level signaling protocols are also provided, in particular for GPRS radio resource management (GRR), GPRS mobility management (GMM), session management (SM), etc. . . . .
In particular, in the radio resource management protocol, various modes are possible in packet mode for a mobile station:                a “packet transfer” mode in which resources are allocated temporarily when data actually needs to be transmitted during a call, these resources forming a temporary virtual channel known as a temporary block flow (TBF) channel enabling data to be transferred between the mobile station and the network in a given transmission direction; and        a “packet idle” mode in which no TBF is set up.        
In contrast, in circuit mode, the mode in which resources are allocated to a mobile station is known as a “dedicated” mode, these resources then being dedicated resources that are allocated to the mobile station for the duration of the call.
In particular, in the mobility management protocol, various procedures are provided whereby, during a GPRS session in a “ready” state, a mobile station MS informs the SGSN of any change of cell by means of a cell update message, whereas in a “standby” state, a mobile station MS informs the SGSN of any change of routing area (RA) by means of a routing area update message (where a routing area corresponds to a set of cells). Similarly, in an “idle” state, outside a GPRS session, the location of the mobile station MS is updated if the mobile station changes routing area. These procedures relating to the mobility management protocol thus enable mobile stations to be located to some extent (i.e. sufficiently for the operating needs of the system), but with accuracy that is well below that provided by location services, since at best they can do no more than indicate the cell in which the mobile station is located.
For packet mode services, location services are implemented as follows, this description being given with reference to FIGS. 2 to 5 which are taken from above-cited specification 3GPP TS 43.059.
Location service support entities are provided, such as, in particular in the access network, a serving mobile location center (SMLC) entity whose function, on receiving a transmission request from a call network entity is to coordinate the various actions required for delivering said location services, such as: requesting the radio measurements needed for locating the mobile station MS, transmitting the results of these measurements to a suitable calculation function, receiving the result from said calculation function, and forwarding the result in response to the received request.
As summarized in FIG. 2, for a mobile station MS that is to be located (a target mobile station):                in a step 1, the SGSN sends a request to the BSS to locate the mobile station MS in a “BSSGP perform location request” message;        in a step 2, a location procedure is implemented, this procedure involving an exchange of signaling between the BSS, SMCL, and MS entities; and        in a step 3, the BSS sends to the SGSN its response to the current location request in a “BSSGP perform location response” message.        
FIG. 3 summarizes the location procedure step:                in a step 4, the BSS sends a request to the SMLC in a “BSSAP-LE perform location request” message;        in a step 5, the SMLC coordinates the various actions required, including in particular sending the necessary signaling messages; and        in a step 6, the SMLC sends its response to the BSS in a “BSSAP-LE perform location response” message.        
FIG. 4 summarizes the protocol layers used for supporting the corresponding signaling between the MS and the SMLC. There can be seen in this figure, in order:                in the mobile station MS:        an RF GSM layer;        a medium access control (MAC) layer;        a radio link control (RLC) layer;        a logical link control (LLC) layer;        a tunnelization of messages (TOM) layer; and        a radio resource LCS protocol (RRLP) layer;        in the BSS, at the “Um” interface with the mobile station MS:        an RF GSM layer;        a medium access control (MAC) layer; and        a radio link control (RLC) layer;        in the BSS, at the “Gb” interface with the SGSN:        an L1bis layer;        a network service layer; and        a BSS GPRS protocol (BSSGP) layer;        in the SGSN at the “Gb” interface with the BSS:        an L1bis layer;        a network service layer;        a BSS GPRS protocol (BSSGP) layer;        a logical link control (LLC) layer; and        a tunnelization of messages (TOM) layer;        in the BSS, at the “Gb” interface with the SGSN:        an L1bis layer;        a network service layer; and        a BSS GPRS protocol (BSSGP) layer;        in the BSS at the “Lb” interface with the SMLC:        a message transfer part (MP) layer;        a signaling connection control part SCCP) layer;        a base station system application part—LCS extension (BSSAP-LE) layer; and        a base station system application part (BSSLAP) layer; and        in the SMLC at the “Lb” interface with the BSS:        a message transfer part (MTP) layer;        a signaling connection control part (SCCP) layer;        a base station system application part—LCS extension (BSSAP-LE) layer;        a base station system application part (BSSLAP) layer; and        a radio resource LCS protocol (RRLP) layer.        
Over the “Gb” and “Um” interfaces, the corresponding signaling uses the signaling resources of the GSM/GPRS system. Over the “Lb” interface between the SMLC and the BSS, the corresponding signaling uses specific resources provided specifically for these location services.
A procedure is also provided to keep an SMLC informed of the cell changes of a mobile station while executing a location procedure. Such a procedure is summarized in FIG. 5:                in a step 7, the mobile station MS detects a change of cell, and as appropriate it sends a cell update message or a routing area update message to the SGSN;        in a step 8, the SGSN sends the BSS a data unit or message known as a “BSSGP FLUSH-LL”, where such a message is defined in specification 3GPP TS 48.018; this enables LLC PDU data units to be rerouted in the down direction (i.e. from the network to mobile stations) from an old cell (prior to cell change) identified by information BVCI(old) to a new cell (after change of cell) identified by an information BVCI(new), where BVCI stands for “BSSGP virtual connection identifier”;        in a step 9, the BSS sends the SGSN an acknowledgment of receipt in the form of a data unit or message known as “BSSGP FLUSH-LL-ACK” (likewise defined in specification 3GPP TS 48.018); and        in a step 10, the BSS sends the SMLC a “BSSAP-LE perform location information” message, which message contains the identity of the new cell.        
It will nevertheless be understood that a location procedure cannot be pursued under all circumstances, and that sometimes execution of such a procedure needs to be aborted by the SGSN.
A problem is thus to enable the SGSN to determine when execution of a current location procedure needs to be aborted, in the event of a change of cell.
One solution to this problem is proposed in the document: 3GPP TSG GERAN WG2 #6bis, Aix-en-Provence, France, October 22-26, GS-010232, agenda item 7.2.5.9. That solution consists merely in considering that if a change of cell is an inter-NSE change, then the SGSN aborts the current location procedure.
As has been observed by the Applicant, such a solution suffers in particular from the drawback that depending on the manner in which cells are distributed in different NSEs, that can lead to a situation in which such a procedure can be aborted following any change of cell. For example, this can compromise any architecture based on distributed NSEs, in which a relatively large number of NSEs are present per BSS (e.g. because the NSEs are relatively smaller).
A current location procedure needs to be aborted by the SGSN, in particular in the event of a change of cell corresponding to a change of BSS (under such circumstances, the SMLC in charge of the location procedure need not be the same). However, merely knowing the identity of the new cell does not enable the SGSN to determine whether or not the location procedure needs to be aborted. In order to determine whether the procedure should be aborted, it would be necessary for the SGSN to know how the cells are shared amongst different BSSes. Unfortunately, the SGSN cannot see the BSS through the “Gb” interface, since it deals with said interface only via network service entities (NSEs). In addition, the way in which cells are shared amongst different BSSes does not necessarily correspond to the way in which cells are shared amongst different NSEs. In particular, such sharing can be implemented on the basis of operating and maintenance criteria that need not be the same from the access network point of view (for the BSS) and for the core network point of view (for the NSE). There can thus be one or several NSEs per BSS.
Thus, the Applicant has found that the prior solution outlined above can lead to a situation in which such a procedure can be aborted as the result of any change of cell whereas, the real need is to abort execution only when changing BSS.
A particular object of the present invention is to avoid all or some of the above-mentioned drawbacks.